Champagne Nickel Z-Door Zero Clearance Bronze Glass Bifold Tracked Door For Martin LF42U and LF42BG Models. Thermo-Rite's stock Z-Doors are made as inside fits. We use Air Studs™ or an Air Draft Bar™ to elevate the frame so that it maintains the cooling air intakes located inside the firebox. The Z-Door is a sleek, full-framed, tarnish free anodized aluminum enclosure.